A foam is a substance that is formed by trapping many gas bubbles in a liquid or solid. The term foam may also refer to anything that is analogous to such a phenomenon, such as quantum foam, polyurethane foam (foam rubber), EPS (Expanded Polystyrene) foam, Polystyrene, or many other manufactured foams. Fine foam can be considered a type of colloid. Syntactic foam is foam produced of composite materials synthesized by filling a matrix with hollow particles or shells. The matrix material can be selected from any suitable metal, metalloid, polymer, or ceramic. A wide variety of hollow shells are available, including cenospheres, glass microspheres, and carbon and polymer microballoons, hollow ceramic and metal or metalloid shells. The presence of hollow shells results in lower density, higher strength, a lower coefficient of thermal expansion, and, in some cases, radar or sonar transparency.
The compressive properties of syntactic foams primarily depend on the properties of the hollow shells, whereas the tensile properties depend on the matrix material that holds the shells together.
Customization is one of the biggest advantages of syntactic foams. There are several ways of adjusting the properties of these materials. One method is to change the volume fraction of hollow shells in the syntactic foam structure. A second method is to use hollow shells of different wall thickness. A third method is to adjust the geometrical shape of the shells. In general, the compressive strength of the material is proportional to its density.
Syntactic materials were developed in early 1960s as buoyancy aid materials for marine applications. Other characteristics led these materials to aerospace and ground transportation vehicle applications. Current applications for syntactic foam include buoyancy modules for marine riser tensioners, boat hulls, deep-sea exploration, autonomous underwater vehicles (AUV), parts of helicopters and airplanes, and sporting goods such as soccer balls.
Metallic foams are known in the prior art. Metal foams are a class of materials with very low densities and novel mechanical, thermal, electrical, and acoustic properties. In comparison to conventional solids and polymer foams, metal foams are light weight, recyclable, and non-toxic. Metal foams offer high specific stiffness, high strength, enhanced energy absorption, sound and vibration dampening, and tolerance to high temperatures. By altering the size and shape of the cells in metal foams, mechanical properties of the foam can be engineered to meet the demands of a wide range of applications.
Various methods are known in the art for preparing metallic foams. According to one method, metal powders are compacted together with suitable blowing agents, and the compressed bodies are heated above the solidus temperature of the metal matrix and the decomposition temperature of the blowing agent to generate gas evolution within the metal. Such “self-expanding foams” can also be prepared by stirring the blowing agents directly into metal melts. Metallic foams can also be prepared as “blown foams” by dissolving or injecting blowing gases into metal melts. Metallic foams can also be prepared by methods wherein gasses or gas-forming chemicals are not used. For example, metal melts can be caused to infiltrate porous bodies, which are later removed after solidification of the melt, leaving pores within the solidified material.
Metallic foams of this type have been shown to experience fatigue degradation in response to both tension and compression. Plastic deformation under cyclic loading occurs preferentially within deformation bands, until the densification strain has been reached. The bands generally form at large cells in the ensemble, mainly because known processes for producing these materials do not facilitate formation in a uniform manner. Such large cells develop plastically buckled membranes that experience large strains upon further cycling and will lead to cracking and rapid cyclic straining. As a result, the performance of existing foams has not been promising due to strong variations in their cell structure as disclosed in Y. Sugimura, J. Meyer, M. Y. He, H. Bart-Smith, J. Grenstedt, & A. G. Evans, “On the Mechanical Performance of Closed Cell Al Alloy Foams”, Acta Materialia, 45(12), pp. 5245-5259, incorporated herein by reference.
In the production of closed cell metallic foams, one obstacle is the inability to finely control cell size, shape, and distribution. This makes it difficult to create a consistently reproducible material where the properties are known with predictable failure. One method for creating a uniform closed cell metallic foam is to use prefabricated spheres of a known size distribution and join them together into a solid form, such as through sintering of the spheres, thereby forming a closed cell Hollow Sphere Foam (HSF).
In addition to metal foams formed with a blow gas, there are also syntactic metal foams. In this case, a metal matrix surrounds a hollow shell composed of glass, ceramic, metal, or metalloid.
U.S. Pat. No. 4,568,389 (Torobin) describes a structure and method of producing closed cell metallic foam using hollow metal shells mixed into a metal matrix.
In 1998 a published report to the Office of Naval Research entitled “Fabrication and Microstructure of Metal-Metal Syntactic Foams”, Dr. Nadler et al. further describes a metal-metal syntactic foam matrix composite microstructure consists of thin-wall, hollow Fe—Cr stainless steel spheres cast in various metal matrices including aluminum alloys 6061, 7075, 413, magnesium alloy AZ31B, and unalloyed aluminum and magnesium. Stainless steel spheres fabricated by the team were sufficiently uniform to allow arrangement into random or periodic arrays. These arrays were infiltrated by an aspiration casting process, resulting in hollow shells with an interstitial metal matrix. During their research the team reported intermetallic formation on the boundaries of the hollow shells due to interaction of iron of the sphere and the alumina of the matrix.
U.S. Pat. No. 7,641,984 (Rabiei) discloses metal spheres in a metal matrix fabricated with powder processing and casting techniques. This patent does not sufficiently address the problem of intermetallics forming between the matrix material and shell material. Additionally, the proposed method does not optimize the interstitial matrix. In this case, shells that are in contact with other shells induce voids resulting in reduced strength.